Grout Problems

[kak]

I'm in the process of retiling my bathroom, 450x300 padova tiles with 3mm spacers. The gout I was recommended was bal superflex wide joint limestone colour. I have now grouted half the walls and this looks fine but when it gets wet it discolours. This later dries out but in the meantime it looks a mess. On contacting bal they say it's due to the grout being a cement based one. I am worried at using this in the shower enclosure on how it will look and possibly wear. Has anyone any ideas - I really don't want to rake out the stuff I've already put up but don't think I can live with the wetting appearance. The supplier has been less than helpful on this - saying they have never had any problems etc...